USDA Forest Service. 1979. A guide to common insects and diseases of forest trees in the northeastern United States. Northeast. Area State Priv. For., For. Insect and Disease Management., Broomall, PA. p. 123, illus.

Beech bark disease is an insect-fungus 'complex" that kills or injures American beech. Disease results when a species of Nectria, especially N. coccinea var. faginata, infects the bark of the trunk through feeding wounds made by the scale insects Cryptococcus fagisuga and Xylococculus betulane. Within the range of the disease, few large beeches escape injury. In 1975, the "killing front" was in eastern Pennsylvania and western New York, the disease having spread about 500 miles since it was first detected on the East Coast in 1929. The spread continues westward and southward.

The beech scale, C. fagisuga, usually infests trees more than 8 inches in diameter, but it is also found on smaller trees. An individual C. fagisuga, is tiny, but the insect secretes a white, woolly wax covering that can be seen easily. The insect may cluster in cracks or under branches, or may cover the entire trunk. The scale reproduces asexually (no males are known). Eggs hatch in late summer, and the larvae crawl and disperse. Wind and perching birds help to spread the insects. During winter the scales lose their legs, secrete their protective covering, and pierce the bark to feed. Thereafter they do not move. The insects can no longer live where the fungus has killed the bark.

X. betulae has a life cycle similar to that of C. fagisuga. The scale is red-orange, but after it becomes stationary in a crevice, it secretes a white wax to cover itself. The scale may be detected by hair-thin tubes that project form crevices in the bark. These insects produce additional wounds for the fungus to invade.

The asexual stage of Nectria produces a cushion of white spores during wet weather in the summer. These cushions resemble the woolly secreations of C. fagisuga. The small, red fruiting bodies of the sexual stage are found in clusters. They mature in the fall and can be seen best then.

Fungal invasion is apparent 3 to 5 years after the scale insects appear. Dark, dead patches are produced where other fungi, such as species of Hypoxylon, enter. Nectria infection points may become walled off with callus tissue, giving the tree a "pock-marked" appearance. Some trees are killed within a year after Nectria is apparent; others linger for several years. The leaves of the latter turn yellow early in the growing season, and the crowns are thin. Trees that survive a first attack may become reinfested and die later. A few trees seem to be resistant to scale infestation.